1. Field of the Invention
The invention relates in general to electrical inductive apparatus, such as power transformers, and more specifically, to new and improved insulative mechanical support structures in such apparatus.
2. Description of the Prior Art
The distribution of an A.C. voltage, and thus the dielectric stress, across two insulating materials or dielectrics in series, divides inversely with their dielectric constants. Thus, for an insulating structure in which two dielectrics are in series, it would be ideal if the material having the lowest dielectric strength would have the highest dielectric constant. The high dielectric constant material would transfer electrical stress into the lower dielectric constant material, which would have the dielectric strength to accommodate the higher electrical stress. Each material would be stressed according to its capability, and the dimensions and clearances of the insulating structure could be optimized to construct electrical inductive apparatus, such as power transformers and electrical reactors, smaller and lighter, thus lowering their cost.
Since solids generally have a higher per unit dielectric strength than gases or liquids, the preferred dielectric constant of solid insulation, when placed in series with a gas or liquid dielectric, should be lower than the dielectric constant of the associated gas or liquid. Unfortunately, available solid materials have dielectric constants which are opposite to that desired, i.e., 4 to 6 for solids, close to 1 for gases and vapors, and close to 2 for insulating liquids, such as mineral oil. Thus, the insulating structures which make up an electrical power transformer are designed accordingly, using insulative shapes and dimensions which will not exceed the electrical breakdown strengths of the various insulating materials utilized.
Liquid filled electrical inductive apparatus, in general, has excellent economically attractive materials available, such as mineral oil which has a dielectric constant in the range of 2.0 to 2.2, and Kraft paper, which when soaked with mineral oil, has a dielectric constant of about 3.75. While liquid filled electrical inductive apparatus may be reduced in size and weight by using a solid having a dielectric constant lower than that of oil soaked Kraft paper, any substitute for this solid insulation must not offset the potential cost savings, by its own higher initial cost. In addition, it must have the necessary physical characteristics, electrical strength, and chemical compatiblity essential to enabling such apparatus to operate for many years at an elevated operating temperature, while being subjected to lightning and switching surges on the connected electrical transmission and distribution lines.
Various substitutes for cellulose in liquid filled electrical inductive apparatus have been found, using certain organic plastic materials. For example, U.S. Pat. No. 3,611,225, which is assigned to the same assignee as the present application, discloses solid insulation formed of an organic resin binder, filled with an organic resin filler, with the latter being selected to lower the dielectric constant of the resulting composite structure. Materials are disclosed which will lower the dielectric constant of the solid to 2.7, for example. U.S. Pat. No. 3,775,719, which is also assigned to the same assignee as the present application, discloses solid insulation for liquid filled electrical inductive apparatus having a dielectric constant in the range of 2 to 2.7. The solid insulation is selected from at least one of the group consisting of thermosetting cross-linked 1, 2-polybutadiene hydrocarbon resins and copolymers thereof, and at least partially crystalline isotactic polystyrene. U.S. Pat. Nos. 3,683,495 and 3,720,897, which are also assigned to the same assignee as the present application, disclose the use of solid and foamed plastic materials, respectively, to form certain insulative structures which also have certain mechanical advantages, in liquid filled electrical inductive apparatus.
Even though inventions have been made which could replace cellulose in liquid filled transformers, the dielectric constant of oil impregnated cellulose is acceptable, oil impregnated cellulose has a very high electrical breakdown strength, and it is economically attractive. Thus, there has been no strong pressure to deviate from the long established use of cellulose in electrical power apparatus of the liquid filled type.
In the past, when liquid filled electrical inductive apparatus was to be operated near building and/or people, they have been constructed with a polychlorinated biphenyl liquid (PCB), instead of mineral oil, because of the fire resistant character of the PCB's. The Federal Toxic Substance Control Act, passed in 1976, has made it mandatory that the use of PCB's in such apparatus be phased out over a short period of time. The relatively high cost of substitute liquids has focused attention on alternatives, such as the use of air cooled, gas cooled, and gas/vapor cooled transformers. The design of such gas, and/or gas/vapor transformers in voltage and KVA ratings required to replace PCB filled apparatus, with adequate mechanical short circuit strengths, and costs which are competitive with PCB filled transformers, has highlighted the fact that a need exists for a low cost insulating material for use in gas or gas/vapor, which has a dielectric constant of 3.5 or less, and preferably 2.5 or less, as the gases and gas/vapor combinations have a dielectric constant of near 1. Known insulating materials which have a relatively low dielectric constant, such as wholly aromatic polyamide paper (Nomex), are very costly, making it difficult to provide a gas or gas/vapor transformer which is competitive costwise with liquid filled electrical inductive apparatus, which may conveniently use cellulose.
A high dielectric constant solid material which penetrates a non-uniform or highly stressed dielectric field in a gaseous dielectric, can cause very low corona inception voltages and low dielectric breakdowns, compared with either no solid spacers, or solid spacers which are terminated within a more uniform field. Thus, certain desirable and conventional arrangements of coil and winding supporting spacers, such as those used in liquid filled apparatus, are denied use in gas and gas/vapor applications because of high dielectric constant spacers penetrating non-uniform fields in an insulating dielectric having a dielectric constant of near 1. In addition to meeting the dielectric constant and cost factors, the insulating material must also have the high mechanical strength, both compressive and flexural, necessary to withstand short circuit forces, and it must be compatible thermally and chemically in its intended operating environment.